I don't know if the warnings have anything to do with it, but this code:

  1 use Device::USB;
  2 
  3   $usb = Device::USB->new();
  4   $dev = $usb->find_device('0403', '6001');
  5   printf("Device: %04X:%04X\n", $dev->idVendor(), $dev->idProduct(
+));
  6   $dev->open();
  7   print "Manufactured by: $dev->manufacturer()\n";
  8   print "Product: $dev->product()\n";
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gives the concatenation warnings plus:

Can't call method "idVendor" on an undefined value at kk.pl line 5, <D
+ATA> line 1.
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Here is the part of dmesg that applies

[349416.436284] usb 4-2: new full-speed USB device number 3 using uhci
+_hcd
[349416.634282] usb 4-2: New USB device found, idVendor=0403, idProduc
+t=6001
[349416.634294] usb 4-2: New USB device strings: Mfr=1, Product=2, Ser
+ialNumber=3
[349416.634302] usb 4-2: Product: FT232R USB UART
[349416.634309] usb 4-2: Manufacturer: FTDI
[349416.634316] usb 4-2: SerialNumber: A100X16Z
[349416.642482] ftdi_sio 4-2:1.0: FTDI USB Serial Device converter det
+ected
[349416.642595] usb 4-2: Detected FT232RL
[349416.642603] usb 4-2: Number of endpoints 2
[349416.642610] usb 4-2: Endpoint 1 MaxPacketSize 64
[349416.642618] usb 4-2: Endpoint 2 MaxPacketSize 64
[349416.642624] usb 4-2: Setting MaxPacketSize 64
[349416.644397] usb 4-2: FTDI USB Serial Device converter now attached
+ to ttyUSB0
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OK, SLAM ON THE BRAKES: I jumped onto my Ubuntu 13.04 box and ran the same code. It asked for Device::USB. I installed it correcly by using apt-get install libdevice-usb-perl. The warnings went away and all I still have left is the error: "Can't call method "idVendor" on an undefined value at kk.pl line 5, <DATA> line 1." which is probably due to my lack of knowledge so I have to go study this. Thanks for your help and the lesson here is: IF YOU'RE USING UBUNTU, AVOID CPAN!!"